Realcat!plagg AU idea except I don't have the energy to draw a comic of it so you have to read a wall of text instead.
There is still magic and there are still kwamis. Kwamis can take a variety of forms. Tikki prefers to stay in the regular little sprite form we're familiar with on the show. Marinette gets her miraculous the same way she does in canon. Nothing on Marinette or Tikki's end deviates from canon, actually.
Adrien, on the other hand, gets his miraculous after a ratchet stray black cat wanders into his room carrying a strange jewelry box. He's mainly concerned with trying to get this cat out of his room without his father noticing, but after a while of trying and failing to capture it, he resigns himself to fiddling with the jewelry box and the ring inside of it. He discovers by accident that it's a magic ring that gives him superpowers. He forgets about the cat in his room (which is no longer in his room but in his ring) and goes off galavanting as a superhero the way he does in canon.
He's smart enough to deduce that the ring and his new fugly little cat are connected (since the ring gives him a black cat persona), but he has no reason not to think that the cat, now named Plagg (after the sound of him throwing up a hairball as per @buggachat 's comic, but also happening to be Plagg's actual real name, which Plagg finds hilarious but is also a little offended by) is anything other than a normal but very shady cat. Yeah, he's somehow connected to this magic ring, but he's just a normal, lazy cat who does normal, destructive cat things and shows no signs of higher intelligence. Adrien's conclusion is that as a counterbalance to becoming a superhero, he also now has to take care of this weird fleabag as a way to keep him humble. But he doesn't mind after a while.
Ladybug and Cat Noir don't talk about their kwamis that much at first. They're still getting the hang of the superhero thing and figuring out what's safe for them to talk about. But slowly they get more comfortable talking about different things, and that's when Cat Noir brings up the origins of their powers.
"So what happened when you got your miraculous? Was it just one ladybug bringing you your earrings, or like a whole swarm?"
"...What? It was my kwami."
Adrien goes home that night and immediately demands answers from his cat. Plagg gaslights him for a while by remaining a normal, oblivious cat. Adrien eventually falters, thinking he's losing his mind, and ceases throwing accusations at his cat. And it is at this point that Plagg finishes the bit, finally transforms into his canon kwami form, and laughs at Adrien, who is screaming
